This invention relates to a single mode resonant cavity, and particularly, though not exclusively, to cavities for heating exhaust gas catalytic converters.
The increasing importance of environmental issues in many countries in recent years has led to the development of ways to minimize pollution from sources such as vehicle exhausts. This has derived both from legislation and from a desire on the part of companies to be seen to be concerned with minimizing harmful environmental impacts. One important technology which has been developed is that of the catalytic converter. In such systems the exhaust gasses are made to pass over a heated catalyst matrix in a chamber in the presence of other reactive species. Although such systems work very well in general, they only work at peak efficiency after the catalyst has been heated above a critical temperature known as its "light off" temperature. As the catalyst in most systems is heated mainly by the exhaust gas itself, there will be an initial period of time when the exhaust gas is poorly converted. This problem is particularly severe in countries having cooler climates such as Japan, USA, Canada, Russia, Korea, and Northern Europe.
One solution to this problem is to heat the catalyst using a different method which does not depend upon the temperature of other parts of the engine. A known way of heating catalyst mixtures in catalytic converters using microwave radiation is disclosed in WO 90/14507. In this method microwave energy is used to heat a part of a matrix of catalyst material placed in a waveguide which need not be a resonant cavity. However, this method has the drawback that if a resonant cavity is employed then it is likely to heat up together with the catalyst and expand. This expansion alters the resonant frequency of the cavity which then fails to match the microwave energy source electrically.
According to a first aspect of the invention there is provided a single mode resonant cavity, as claimed in claim 1. This can have the advantage that as the cavity warms up and physically expands the coupling efficiency between the cavity and a narrow waveband microwave source remains good.
According to a second aspect of the invention there is provided a catalytic converter system for exhaust gas, as claimed in claim 5.
According to a third aspect of the invention there is provided a method of coupling energy via a single excitation mode to a resonant cavity from a source of a.c. electromagnetic energy having a given bandwidth, comprising adapting the cavity to absorb energy via the single excitation mode in a plurality of absorption bands having different respective resonant frequencies. The cavity is preferably adapted by either the provision of an electrically conductive element inside the cavity adjacent and electrically connected to a wall thereof, or by deformation thereby providing at least a part of the cavity with an elliptic cross-section.